Method of gas plating adherent coatings on silicon



Nov. 23, 1965 METHOD OF GAS PLATING ADHERENT COATINGS ON SILICON W. C.JENKIN Fil ed June 25, 1962 Si/icon Wafer Efched wiff; Acid R/nsed W/fhWafer and Air Dried Placed in 605 P/af/na Chamberand heafed f0 6619 EPlating Chamber flushed W/f/l m/Xfure of H C!- gas and Hydrogen Pur edMW? h Or en five/1 flushed M'Z'MIV H 6a: P/azed using 90560115 mmure60/7/0/17/779 Me fa/ Carbonyl Sf/fcvn Wafer M777 adhere/72 coaf/agofmefa/ INVENTOR W/LL/AM c. dENK/N BY M m ATTORNEKfi United StatesPatent 0 F 3,219,432 METHOD OF GAS PLATING ADHERENT COATINGS 0N SILICONWilliam C. Jenhin, Dayton, Ohio, assignor to Union Carbide Corporation,a corporation of New York Filed June 25, 1962, Ser. No. 205,050 4Claims. (Cl. 117-213) This invention relates to a method of obtainingadherent plating deposits on oxidized substrate surfaces and materialswhich are difiicult or impossible to plate satisfactorily by ordinaryplating methods. The invention is more particularly concerned with aprocess of obtaining an adherent coating on such materials as depositedby thermal decomposition of a gaseous heat-decomposable compound, andcommonly known as gas plating.

It has been observed that merely cleaning of a material by conventionalmethods and then subjecting the thus cleaned material to heat in thepresence of vaporized heat-decomposable compounds or mixtures of suchcompounds does not produce an adherent coating deposit. Many industrialmaterials and parts are required to be provided with an adherent coatingand which has been difiicult to achieve. For example, materials whichreadily oxidize in the presence of air or oxygen are very difficult orimpossible to plate by ordinary methods, and especially materials which,after cleaning, rapidly form non-reducible oxides. Thus, in themanufacture of silicon rectifiers and diodes, a coating is requiredwhich is strongly adherent to Withstand soldering or brazing ofconductive metals thereto, and to remain adherent throughout the usefullife of the product. Utilizing the method of this invention, anadherent, brazable metal deposit is obtained on these difficultycoatable materials.

Test results carried out in connection with the method of the presentinvent-ion have indicated that there are two critical requirements thatmust be observed in order to obtain an adherent deposit of metal ormetallic compound when gas plating such deposits on a substrate surfacewhich readily acquires an oxide film. These requirements are:

(l) The material must be cleaned completely free of non-volatile salts,oxides, etc., even of a mono-molecular film, and kept free ofcontamination or reoxidation until the gas plating of the same isaccomplished.

(2) The surface of the material thus cleaned must be treated with a gasthat is sutficiently adsorbed to cause desorption of other andinterfering gases. Such adsorbed gas, however, is one that is readilydesorbed as gas plating and deposition of a coating is initiated, ratherthan at a later period when it tends to form a barrier to good adhesion.

In carrying out the process, the same can be made operable at ordinaryatmospheric pressure conditions, or employing sub-atmospheric or evenpressures above atmospheric. The pressure is thus not a criticallimitation of the process of this invention.

In accordance with the present invention, materials that are difficultto plate because their surfaces immediately acquire a stable anddifiicult reducible oxide are first cleaned by etching the surface withacid or reagents that dissolve a portion of the material, includingoxide or surface film. The operation must be done in the absence ofoxygen to prevent reoxidation of the etched surface. Examples ofmaterials which may be so cleaned and gas plated are silicon andtitanium.

3,219,482 Patented Nov. 23, 1965 The preferred method of cleaningconsists in placing the material in an airtight chamber, then purgingthe air therefrom with a non-oxidizing or non-reactive gas, such ashydrogen or helium. Thereafter the material is heated to 400 to 1000 F.,preferably about 600 F., while retained in the gas plating chamber, andthe heated material subjected to a gaseous mixture of the purging gasand the vapor of a gas that chemically attacks the material and forms avolatile compound as a by-product. Gases which are useful for thispurpose are the halides of hydrogen, gaseous hydrogen chloride being themost economical and easiest to handle.

The process of the present invention is illustrated by the fiow sheet onthe drawing. No vacuum or elaborate equipment is required, asaforementioned, although use may be made of a blower or sub-atmosphericor pressure creating means to cause the gaseous substances to flow orcirculate in direct contact with the material being gas plated.

Etching of the material with hydrogen chloride is effective employing atreatment of one or two minutes and which removes a thin layer of thematerial whereby a fresh and chemically clean substrate surface isproduced.

The etching gas is then purged from the plating chamber with hydrogen orhelium or other non-reactive gaseous mixture that is readily desorbableduring the initial gas plating operation.

Further treatment of the surface preparatory to depositing a coating,consists in exposing the material to a gas that becomes adsorbed anddisplaces other gases that may be present, but is readily desorbed asplating is initiated. For this purpose, ammonia gas is a very effectivegas, particularly in gas plating adherent coating on silicon. Inpractice, the purging gas hydrogen is discontinued, and ammonia admitteduntil the chamber is flushed. The material being gas plated may beallowed to cool temporarily, but preferably it is kept hot. The materialis brought to plating temperature, if this is different from the vaporetching temperature, and a small amount of the vapors of the platingcompound admitted to the gas plating chamber. To prevent prematuredecomposition of the plating compound, a small percentage amount of oneof the gaseous products of decomposition of the plating compound isinjected simultaneously into the gas plating mixture. Such additionpreferably amounts to about 5% by volume of the desorbing gas, e.g.,fresh carbon monoxide.

An example of a suitable plating compound is nickel carbonyl, and inwhich preferably is introduced 0.1% by volume of ammonia. Into thisplating gas mixture is bled approximately 10% by volume of carbonmonoxide, to suppress premature decomposition of the nickel carbonyl.

Once a thin flash coating of metal is deposited, the introduction ofammonia or equivalent desorbing gas is discontinued and inert carriergas or gases introduced in place thereof, together with increasedamounts of the vapors of the plating compound. The temperature of thegas plating is also lowered about two hundred degrees after the initialflash coating and the plating continued.

The strength of the adhesive bond of coatings deposited in accordancewith this process where they are solderable or brazable, are readilydetermined by soldering or brazing a stud on directly opposing surfacesof a plated piece. These are then gripped in a tensile test machine andpulled until a break occurs.

Tests carried out with silicon wafers gas plated in accordance with themethod of the present invention have been found to adhere to the siliconsurface so tenaceously that bits of silicon continue to stick to thecoating deposit gas plated thereon. Thus, the bond between the siliconwafer and the deposite film approaches the strength of the silicon orbase material.

The following examples are exemplary of several embodiments of theinvention, but are not limitative thereof.

Example 1 A thin disc or wafer of silicon is etched free of oxides byimmersing the wafer in an aqueous solution of hydrofluoric acid Thewafer is removed from the solution, rinsed with water and then suspendedin an airtight chamber. Heat is applied to the wafer by an infra-redheat source focused thereon which passes through transparent walls. Thechamber is purged with hydrogen and then flushed with hydrogencontaining 10% by volume of dry hydrogen chloride gas and the siliconwafer heated to 600 F. for two minutes. Thereafter the chamber is purgedwith hydrogen until free of hydrogen chloride, then flushed withammonia. While the wafer is still heated to 600 F., a gaseous mixture,by volume, of 5% carbon monoxide, 0.1% nickel carbonyl vapor, and thebalance ammonia, is injected into the plating chamber for one minute.Then a mixture of carbon monoxide only, and containing 0.2% nickelcarbonyl, is circulated and in contact with the heated wafer for threeminutes. Following the flash coating deposit produced, the temperatureof the wafer is decreased to 400 F., or thereabou-t, and the amount ofnickel carbonyl introduced raised to increase the rate of deposition.The deposited nickel tenaceously adheres to the silicon surface.

Example 2 In this example, an aluminum alloy sheet or panel (3 x 6") isetched in an acid solution comprising a mixture of nitric andhydrofluoric acids (8:1 by volume). After immersing the alloy sheet inthis acid solution for three minutes it is removed and rinsed in cleanwater, then heated and subjected to gas plating as described inExample 1. Using the nickel carbonyl plating gas mixture, the depositednickel is sufiiciently adherent'to withstand a bend test of 180 around adiameter equal to approximately twice the thickness of the plated panel.

Example 3 A titanium alloy bolt is etched free of oxides in an acidmixture of nitric acid and hydrofluoric acid as described in Example 2.After rinsing and drying the same, the etched bolt is suspended in a gasplating chamber and plated as described. In this instance, the chamberis purged with helium, and then flushed with helium containing 5% byvolume of hydrogen bromide, and the bolt heated to 600 F. for twominutes. The chamber is thereafter purged with helium, while holding thebolt heated to 600 F., and a gaseous mixture of helium containing 1% byvolume of tri-isobutyl aluminum introduced into the chamber andcirculated in contact with the heated pretreated bolt.

After approximately ten minutes gas plating treatment, a thin adherentflash coating of aluminum is deposited on the bolt surface. The gasplating rate is then increased by increasing the concentration oftriisobutyl present in the helium tri-isobutyl aluminum plating gas andlowering of the temperature to approximately 500 F.

Having described my process and given preferred embodiments thereof,what is claimed is:

1. A method of gas plating adherent coatings on silicon by thermaldecomposition of gaseous compounds, and which comprises the steps ofetching the surface of the silicon with acid to present a clean surface,subjecting the resultant silicon surface while in the absence of oxygento a gaseous atmosphere containing hydrogen halide gas while heatingsaid silicon, withdrawing the gaseous atmosphere while maintaining thesilicon free of oxidation, thereafter treating the surface by exposingthe same to a gaseous medium containing gas which is adsorbed butreadily desorbed during initiation of said gas plating, and exposing thesaid surface of the heated silicon to a gaseous mixture containing lessthan 10% by volume of a gaseous heat-decomposable compound and whereinthe temperature of said silicon surface is maintained high enough tocause said heat-decomposable compound to decompose and deposit a coatingonto said silicon surface, and wherein said desorbable gas is ammonia.

2. A method of gas plating adherent coatings on silicon by thermaldecomposition of gaseous compounds, and which comprises the steps ofetching the surface of the silicon with acid to present a clean surface,subjecting the resultant silicon surface while in the absence of oxygento a gaseous atmosphere containing hydrogen halide gas while heatingsaid silicon, withdrawing the gaseous atmosphere while maintaining thesilicon free of oxidation, thereafter treating the surface by exposingthe same to a gaseous medium containing gas which is adsorbed butreadily desorbed during initiation of said gas plating, and exposing thesaid surface of the heated silicon to a gaseous mixture containing lessthan 10% by volume of a gaseous heat-decomposable compound and whereinthe temperature of said silicon surface is maintained high enough tocause said heat-decomposable compound to decompose and deposit a coatingonto said silicon surface, and wherein said desorbable gas as ammonia,and said decomposable gaseous metal compound is nickel carbonyl, wherebynickel metal is deposited.

3. A method of gas plating adherent coatings on silicon by thermaldecomposition of gaseous compounds, and which comprises the steps ofetching the surface of the silicon with acid to present a clean surface,subjecting the resultant silicon surface in the absence of oxygen to agaseous atmosphere containing hydrogen halide gas while heating saidsilicon, withdrawing the gaseous atmosphere while maintaining thesilicon free of oxidation, thereafter treating the surface by exposingthe same to a gaseous medium containing gas which is adsorbed butreadily desorbed during initiation of said gas plating, exposing saidsilicon surface while heated to a mixture of gases containing less than10% by volume of a gaseous heat-decomposable compound together with lessthan 25% by volume of a volatile product of decomposition of saidgaseous mixture, said silicon surface being heated to a temperature highenough to cause thermal decomposition of the heat-decomposable compoundand deposition of a coating on said silicon, and wherein said desorbablegas is ammonia.

4. A method of gas plating adherent coatings on silicon by thermaldecomposition of gaseous compounds, and which comprises the steps ofetching the surface of the silicon with acid to present a clean surface,subjecting the resultant silicon surface in the absence of oxygen to agaseous atmosphere containing hydrogen halide gas while heating saidsilicon, withdrawing the gaseous atmosphere while maintaining thesilicon material free of oxidation, thereafter treating the surface byexposing the same to a gaseous medium containing gas which is adsorbedbut readily desorbed during initiation of said gas plating, exposingsaid silicon surface while heated to a mixture of gases containing lessthan 10% by volume of a gaseous heat-decomposable compound together withless than 25% by volume of a gaseous heat-decomposable compound togetherwith less than 25% by volume of a volatile product of decomposition ofsaid gaseous mixture, said silicon surface being heated to a temperaturehigh enough to cause thermal decomposition of the heat-decomposablecompound and deposition of a coating on said silicon, and wherein saiddesorbable gas is ammonia, and said decomposable gaseous metal compoundis nickel carbonyl, whereby nickel metal is deposited.

References Cited by the Examiner UNITED STATES PATENTS 6 Ostrofsky et a1117227 Cummins 117227 Buck 15617 Vrahiotes et a]. 117107.2 Miller 156-17Jenkin 117107.2 Mocanu 117213 RICHARD D. NEVIUS, Primary Examiner.

1. A METHOD OF GAS PLATING ADHERENT COATINGS ON SILICON BY THERMALDECOMPOSITION OF GASEOUS COMPOUNDS, AND WHICH COMPRISES THE STEPS OFETCHING THE SURFACE OF THE SILICON WITH ACID TO PRESENT A CLEAN SURFACE,SUBJECTING THE RESULTANT SILICON SURFACE WHILE IN THE ABSENCE OF OXYGENTO A GASEOUS ATMOSPHERE CONTAINING HYDROGEN HALIDE GAS WHILE HEATINGSAID SILICON, WITHDRAWING THE GASEOUS ATMOSPHERE WHILE MAINTAINING THESILICON FREE OF OXIDATION, THEREAFTER TREATING THE SURFACE BY EXPOSINGTHE SAME TO A GASEOUS MEDIUM CONTAINING GAS WHICH IS ADSORBED BUTREADILY DESORBED DURING INITIATION OF SAID GAS PLATING, AND EXPOSING THESAID SURFACE OF THE HEATED SILICON TO A GASEOUS MIXTURE CONTAINING LESSTHAN 10% BY VOLUME OF A GASEOUS HEAT-DECOMPOSABLE COMPOUND AND WHEREINTHE TEMPERATURE OF SAID SILICONSURFACE IS MAINTAINED HIGH ENOUGH TOCAUSE SAID HEAT-DECOMPOSABLE COMPOUND TO DECOMPOSE AND DEPOSIT A COATINGONTO SAID SILICON SURFACE, AND WHEREIN SAID DESORBABLE GAS IS AMMONIA.